Conventionally, as a pressure sensor device for detecting pressure fluctuations of gas or liquid, a type that detects fluctuations of an applied pressure as oscillation frequency changes of a sensor section has been used.
As such a conventional pressure sensor device, as shown in FIG. 40 and FIG. 41, one is known structured so that a surface acoustic wave element 104 and a surface acoustic wave element 107 formed of comb-shaped electrodes are formed as elements of a sensor section on a piezoelectric substrate 101, and a region of the surface acoustic wave element 104 is set thinner than the region of the surface acoustic wave element 107 (for example, refer to Japanese Unexamined Patent Publication No. 61-82130).
In the above-described pressure sensor device, when the surface acoustic wave element 104 formed in the thinned region receives pressure, the surface stress of the piezoelectric substrate 101 changes and the acoustic velocity of the surface acoustic wave changes, and the intervals of the electrodes of the surface acoustic wave element 104 also change. Accordingly, the resonance frequency of the surface acoustic wave element 104 changes, and based on this change in resonance frequency, the pressure can be detected.
The pressure sensor device also has a function to compensate the temperature according to a change in resonance frequency of the surface acoustic wave element 107 formed on the same piezoelectric substrate.
However, in the above-described conventional pressure sensor device, the surface acoustic wave element 104 formed on the sensor substrate 101 is exposed to the surface of the sensor substrate 101, and no member is provided to protect this, so that when this pressure sensor device is used in a manner in that pressure is applied to the sensor substrate 101, the surface acoustic wave element 104 is exposed to the air containing moisture, and this causes oxidation corrosion and deterioration of the surface acoustic wave element 104 made of aluminum or the like. In such a case, it may become impossible to normally work the pressure sensor device.
In addition, in the above-described pressure sensor device, there is another possibility that foreign matter adheres to the surface of the surface acoustic wave element 104 exposed to the air during use. In this case, the electrode fingers of the surface acoustic wave element 104 are electrically short-circuited by the foreign matter and their normal resonance characteristics cannot be obtained, and it becomes impossible to normally work the pressure sensor device.
Furthermore, in the above-described conventional pressure sensor device, an oscillating circuit to be connected to the surface acoustic wave element 104 is disposed so as to be separated from the sensor substrate 101, and therefore, it is difficult to downsize the entire structure of the pressure sensor device, and the wiring that connects the surface acoustic wave element 104 and the oscillating circuit is easily influenced by electromagnetic noise, resulting in erroneous operations and lowering in measuring accuracy.